Hermetically sealed lead-in structures for electronic devices comprising metal casing members are of two basic types. In the first type, referred to as a matched seal, both the outer metallic casing member and the metal lead passing through a hole in the casing are composed of metals having similar coefficients of thermal expansion. The annular opening between the outer metal member and the lead-in conductor is filled with a glass having a coefficient of thermal expansion similar to that of the metal members, such that the hermetic seal formed by the case, glass and metal lead is essentially strain-free. A typical prior-art matched seal includes metal case and lead members composed of a low expansion cobalt-nickel-iron alloy, such as KOVAR.RTM. alloy, and a glass seal composed of a borosilicate glass of similar thermal expansion characteristics. U.S. Pat. No. 2,062,335 to Scott describes glass and metal sealing materials of the kind often used for matched seals of this type.
A second type of lead-in seal structure is commonly referred to as a compression seal, because seal integrity is imparted by compressive stresses in the seal arising during sealing. U.S. Pat. No. 2,770,923 to Dalton et al. shows a typical compression seal structure, which includes an outer metal casing member composed of relatively high expansion metal, a lead-in conductor of relatively low expansion metal passing through a hole in the outer casing member, and a glass seal of relatively low or intermediate expansion filling the annular opening between the casing and metal lead.
U.S. Pat. No. 3,035,372 to Mayers describes a hermetically sealed lead-in structure of the glass-metal compression type wherein the outer member is composed of steel and the metallic conductor passing through the opening in the steel outer member is composed of nickel-iron alloy. Compression seals incorporating such materials are presently used in the semiconductor industry to provide headers for transistors, integrated circuits and the like. For this application, steel such as SAE 1010 steel, having an average linear coefficient of thermal expansion of about 125 .times. 10.sup.-7 /.degree. C., is used to form the casing constituting the outer member of the seal, while No. 52 alloy, a nickel-iron alloy containing about 51% nickel and having an average linear coefficient of thermal expansion (over the temperature range 30.degree.-450.degree. C.) of about 96-101 .times. 10.sup.-7 /.degree. C., is used for the metallic conductor. One glass used for forming the seal member between the steel case and the alloy conductor consists of about 66 parts SiO.sub.2, 12 parts BaO, 2.7 parts Al.sub.2 O.sub.3, 2.0 parts B.sub.2 O.sub.3, 6.7 parts Na.sub.2 O, 6.6 parts K.sub.2 O, 2.0 parts ZrO.sub.2, 0.7 parts Li.sub.2 O and 1.6 parts F by weight, and has an average linear coefficient of thermal expansion (0.degree.-300.degree. C.) of about 89 .times. 10.sup.-7 /.degree. C.
Compression seals formed from the above materials exhibit good thermal shock resistance, e.g., thermal shock resistance to at least about 425.degree. C., which means that they may be heated from room temperature to 425.degree. C. in a matter of seconds and subsequently cooled to room temperature without mechanical failure or loss of hermetic seal. Adequate thermal shock resistance is a property required by manufacturers of solid state electronic devices incorporating these seals.
It has recently been proposed for reasons of cost to substitute low-expansion cobalt-nickel-iron alloy conductors for nickel-iron alloy conductors in compression-sealed lead-in structures such as above described. Such conductors, which may be composed, for example, of KOVAR.RTM. alloy, typically have average linear coefficients of thermal expansion (30.degree.-400.degree. C.) in the range of about 46-52 .times. 10.sup.-7 /.degree. C. It was found, however, that the resulting glass-metal compression seals between steel and low-expansion cobalt-nickel-iron alloy did not exhibit thermal shock resistance to 425.degree. C. as did the previous seals.
It is a principal object of the present invention to provide a glass-metal compression seal structure comprising a steel outer member and a low-expansion cobalt-nickel-iron lead-in conductor which withstands thermal shock to at least 425.degree. C. without loss of hermetic properties.
Other objects and advantages of the invention will become apparent from the following description thereof.